May-2020

Pilot plant studies of hydrotreating catalysts

Studies of catalysts for hydrotreating lubricant base oil delivered similar results from conventional and high throughput pilot plants

TIAGO VILELA, GRAHAM ORMSBY, JOSÉ CASTRO and HENDRIK DATHE, Avantium
Andrew Michael Lee Gibbs and MARY ANN ABNEY, Ergon
PAUL ROBINSON, Independent Consultant

Article Summary

Proposed catalyst systems for a lubricant base oil hydrotreater were evaluated with two pilot plant studies. Both studies compared two different catalyst loading schemes – System A and System B – where System A outperformed System B for hydrodesulphurisation (HDS) and hydrodenitrogenation (HDN).

One pilot plant is a conventional unit with a single reactor pilot and an available catalyst volume of above 500ml. The second is an Avantium Flowrence unit with 16 parallel single pellet string reactors (SPSRs), each of which has an internal diameter (ID) of 2.6mm and an available catalyst volume of 1.0ml. In the conventional unit, the catalyst schemes were tested one at a time, without replication, while in the Avantium Flowrence unit, the schemes were tested in parallel – at two different space velocities and in quadruplicate for increased accuracy.

The SPSRs in the Avantium unit were fitted into a commercially available Flowrence XR1 system, which ensures stable and highly accurate control of gas flow, liquid flow, and pressure across all reactors. Performance data like hydrogen consumptions and liquid product properties were determined independently per reactor. For this, the products from each SPSR were collected separately and various offline analyses performed, for instance for distillation, sulphur, nitrogen, and aromatics.

Due to the excellent hydrodynamics,2,10 of the SPSR and sophisticated process control, the Avantium unit achieved high reproducibility, resulting in average deviations of less than 0.2 wtppm for HDS and HDN across the quadruple reactors with the same loading scheme.

Results from the conventional pilot plant corresponded closely to results from the Avantium pilot plant. For Catalyst System A, the relative average deviations were less than 1% for HDS and HDN. For Catalyst System B, all relative HDS deviations and two of three HDN deviations were less than 1%.

These observations indicate transitively that the Avantium unit is a suitable alternative to the conventional pilot plant for the customer’s lube oil hydrotreater. It is in fact preferable if one accounts for the advantages of high throughput technology: parallel testing, lower cost and feed amounts, and increased flexibility on testing more options. Moreover, due to the small scale of testing, safe operation can be accomplished in a laboratory setting which would be difficult to achieve with the same number of reactors at conventional scale.

This particular study considered base oil hydrotreating for a single feedstock, but Avantium equipment and methodology also can be employed to evaluate several feedstocks. Moreover, the same technology has been used successfully to examine other fixed bed catalytic processes, including hydrocracking, hydrodewaxing, catalytic reforming, and hydroisomerisation.

Lubricant base oil classification and preparation routes
Depending on the preparation, lubricant base stocks are classified into different groups.³ Groups I, II, and III are manufactured from paraffinic crudes in refineries. They are commonly called mineral base oils or petroleum base stocks to differentiate them from synthetic base stocks, such as those prepared with polyalphaolefins (PAO, Group IV). Group V stocks include all remaining pale oils (naphthenic base oils), which are manufactured from naphthenic crudes, and other synthetic base materials (see Table 1).

The vast majority of lubricants contain Group I or II base stocks. In 1999, the National Advertising Division of the United States Better Business Bureau declared that automotive lubricants made using Group III base stocks could be labelled ‘synthetic’ due to the very severe processing conditions required to produce them, and because the performance of Group III lubricants was comparable to that provided by PAO. Table 1 compares important properties: sulphur content, saturates content, and viscosity index (VI) for all five major groups.

Figure 1 illustrates different routes for preparing Groups I-III base stocks; note that hydroprocessing plays at least some role in each route. Preparation of Group I base oils entails distillation to set viscosity, solvent extraction to remove aromatics and other low-VI molecules, wax removal – by extraction or crystallisation – to improve cold-flow properties, and finishing, which removes remaining impurities and improves both colour and colour stability. Preparation of Groups II and III stocks, often called premium base stocks, relies extensively on hydroprocessing where aromatics are removed with saturation, and wax is removed either with selective hydrocracking or hydroisomerisation.

‘Plus’ categories are recognised informally for marketing reasons. Group I+ has VIs from 103 to 108. Group II+ has VIs from 111 to 119. Group III+ has VIs >130 for light neutral base stocks, and Group IV+ has VIs from 5 to 15 higher than conventional 1-decene.

Premium base stocks are preferred because they have:
•    Lower viscosity, which increases fuel economy during cold starts and reduces engine friction
•    Lower volatility, which reduces oil losses and, consequently, reduces emissions
•    Improved oxidative and thermal stability
•    Improved lubricant performance across a wide range of temperatures, allowing an engine to crank at sub-zero temperatures and also provide superior lubrication during high temperature operation

Hydroprocessing in lubricant feedstock conversion
Traditional solvent based lube oil plants are designed for a specific range of crude oils due to the inherent limitations of solvent extraction units.⁴ If the aromatics content of a crude oil is too high, aromatics extraction will be a bottleneck, and the distillation, dewaxing, and hydrofinishing sections of the production train will be under-utilised, resulting in a low base stock yield. On the other hand, if the wax content is too low, a wax crystalliser may not function efficiently.⁵

Applying hydroprocessing technologies adds feedstock flexibility by increasing the practical range of crude oil properties.

Figure 2 illustrates important chemical reactions in base oil hydroprocessing. While HDS and HDN remove reactive heteroatoms, which accelerate oxidative degradation, saturation and ring opening convert low VI molecules into high VI molecules. Hydrodewaxing removes waxy n-paraffins by converting them into lighter molecules, such as diesel and naphtha constituents, over catalysts containing ZSM-5 or similar materials.⁶ Isomerisation converts n-paraffins into i-paraffins and by this removing waxy molecules.

One considers a lube stock preparation plant in which aromatics removal is accomplished by saturation, that is hydrodearomatisation (HDA). Up to a point – below the aromatics crossover temperature⁷ and within guidelines for safe operation – HDA can be increased simply by raising the average catalyst temperature in the hydroprocessing unit. The same applies if wax removal is accomplished by hydrodewaxing. Higher temperature is not necessarily beneficial in hydroisomerisation, where cracking is undesirable.⁸

Base oil pilot plant studies
For lube base stock production, modest differences in feedstocks, catalysts, and process configuration can have a major impact on product quality. Refiners and catalyst vendors conduct pilot plant studies to ensure that changes are practical and economically viable. Relevant and scalable test data are required and test results from pilot plant studies should enable the reliable prediction of the performance of commercial scale units.